[exim] no reply to STARTTLS

Hello,

I just installed my mail environment with dovecot and exim4(.50).
Everything works like it should - except STARTTLS with exim.

Exim advertises STARTTLS in the EHLO response but upon sending the
STARTTLS command, nothing happens, it looks some client input is
expected. Not if connected via client nor by hand (telnet).

The TLS related config is

tls_advertise_hosts = *
tls_certificate = /etc/ssl/certs/myserver.pem
tls_privatekey = /etc/ssl/private/myserver.pem

I also tried to use exim.crt and exim.key in the /etc/exim4 directory
(don't know who created them, maybe the debian installer).
I already tried making the certs (resp the keyfile) 777.

tls_verify_hosts and tls_try_verify_hosts are not set (resp. set to some
host that does not exist).

What's annoying is, that no single byte is logged about this problem.

BTW, the myserver cert/key work without problems with dovecot. Any
suggestions?

Regards,
Jürgen